This invention relates to transmission of digital data, and particularly to transmission of data with high data rates over a dispersive medium.
Digital data, comprising either computer information or digitized analog information, such as digital signals derived from telephone channels, is often transmitted over line-of-sight microwave links or via tropospheric scattering links between a transmitter and a receiver. Typical links of this type are shown in simplified form in FIGS. 1 and 2.
FIG. 1 represents a line-of-sight communication link between a transmitting station 30 and a receiving station 32. Transmissions between the transmitter 30 and the receiver 32 follow both a line-of-sight path 34 and one or more time variant additional paths, such as path 36 which results from refractive effects of the atmosphere. There may also be transmission paths which result from ground or sea reflections. In the case of digital data transmission, difficulties arise in such a line-of-sight system where the mean delay of the second path 36 with respect to the first path 34 is greater than 0.02 times the symbol time of the data transmission.
FIG. 2 is a simplified illustration of a tropospheric scattering transmission system wherein a transmitter 38 transmits a signal which is scatterd by the troposphere and received by a remote receiver 40. In this type of system there are usually multiple paths between the transmitter 38 and the receiver 40. There are usually a sufficient number of transmission paths that transmission time differences among the paths cannot be easily defined, but is usually expressed in terms of an RMS delay related to the delay spectrum of the received power. For simplication, the drawing of FIG. 2 shows only two paths, labeled 42 and 44, but those skilled in the art will recognize that these are among a large number of present paths. In a tropospheric transmission system for digital signals, problems arise when the RMS deviation value of the delay spectrum exceeds 0.1 to 0.2 times the symbol time. Accordingly, high data transmission speeds also present difficulties in this type of system.
It has been suggested in a published article that multipath difficulties arising from the transmission of high speed digital data be alleviated by increasing the symbol time through the use of parallel transmission channels. In a paper entitled "System Design and New Techniques For an Overwater 100 Kilometer Span Digital Radio", the authors Yoshiva et al. describe a system wherein a stream of digital data at a rate of 200 megabits per second is divided into four substreams, each of which is transmitted on a different carrier using 16 QAM modulation. The effect of this design is not only to increase the symbol time by the use of the 16 QAM modulation technique, but also to increase the symbol time by providing a slower transmission rate of data on each of four subcarriers. The spectral characteristics of this system are illustrated in FIG. 3b.
If a signal is modulated onto a single carrier with a high data rate, a spectral characteristic of the transmitted signal will be a single broad spectrum about the carrier frequency, as illustrated by curve 46 in FIG. 3a. In the system described by Yoshiva et al., the data is modulated into four subcarriers 48A, 48B, 48C and 48D, each of which has a narrower spectral range because of the lower data rate of the signal. The present invention relates to a transmission system of this type, wherein digital data in a digital data stream is separated into substreams of data at a lower data rate. The substreams are used to separately modulate subcarriers or other component signals which are transmitted in parallel to a receiver at which the substreams are derived from component carriers and reassembled into an output data stream.
It is an object of the present invention to provide an improved transmissions system for the high speed transmission of digital data over a dispersive medium.
It is a further object of the present invention to provide such a system having improved diversity characteristics.
It is a further object of the present invention to provide such a system having a modulator which reduces intersymbol interference caused by components of the system.
It is a further object of the present invention to provide such a system having improved data stream timing techniques to enable the efficient reconstruction of a data stream following parallel transmission of data substreams on different channels.